"It was if I had Novocain," Harper said in the live interview Jan. 21. "I thought what the heck is happening to me?"
"I went to the hospital and they couldn't find much of anything. Now I'm still awaiting test results," she said at the time, also adding, "I'm feeling great."
Harper tells People magazine in its latest issue, on newsstands Friday, that the tests from that January hospital stay revealed she has leptomeningeal carcinomatosis, a rare condition that occurs when cancer cells spread into the fluid-filled membrane surrounding the brain.
According to People, Harper's doctors have told her she may have just three months to live.
"I don't think of dying," the Emmy-winning actress tells the magazine. "I think of being here now."
Harper appeared on "GMA" in January to promote her new memoir, "I, Rhoda." In the book she revealed for the first time that she battled lung cancer in 2009 while starring in the Broadway show "Looped."
"I thought this is just not going to be good for comedy if the audience is out there worrying about their Rhoda," Harper said of her decision to keep her disease, which she fought with a combination of chemotherapy and radiation, private.
Now Harper, who is married with one daughter, has decided to take her current health struggle public.
"There are moments when she feels extremely sad. She's also extremely funny," J.D. Heyman, executive editor of People, told "GMA." "She's very, very happy that she's had the life that she had."